Scrymgeour-Wedderburn J F, Reith C A, Sillar K T
Gatty Marine Laboratory, School of Biological and Medical Sciences, University of St Andrews, Fife, UK.
Eur J Neurosci. 1997 Jul;9(7):1473-82. doi: 10.1111/j.1460-9568.1997.tb01501.x.
The development of intrinsic, N-methyl-D-aspartate (NMDA) receptor-mediated voltage oscillations and their dependence on co-activation of 5-hydroxytryptamine (5HT) receptors was explored in motor neurons of late embryonic and early larval Xenopus laevis. Under tetrodotoxin, 100 microM NMDA elicited a membrane depolarization of around 20 mV, but did not lead to voltage oscillations. However, following the addition of 2-5 microM 5HT, oscillations were observed in 12% of embryonic and 70% of larval motor neurons. The voltage oscillations depended upon co-activation of NMDA and 5HT receptors since they were curtailed by selectively blocking NMDA receptors with D-2-amino-5-phosphonovaleric acid (APV) or by excluding Mg2+ from the experimental saline. 5HT applied in the absence of NMDA also failed to elicit oscillations. Oscillations could be induced by the non-selective 5HT1alpha receptor agonist, 5-carboxamidotryptamine (5CT) and both 5HT- and 5CT-induced oscillations were abolished by pindobind-5HT1, a selective 5HT1alpha receptor antagonist. To test whether 5HT enables voltage oscillations by modulating the voltage-dependent block of NMDA channels by Mg2+, membrane conductance was monitored under tetrodotoxin. Although 5HT caused membrane hyperpolarization of 4-8 mV, there was little detectable change in conductance. NMDA application caused an approximate 20 mV depolarization and an 'apparent' decrease in conductance, presumably due to the conductance pulse bringing the membrane into a voltage region where Mg2+ blocks the NMDA ionophore. 5HT further decreased conductance, which we propose is due to its enhancement of the voltage-dependent Mg2+ block. When the membrane potential was depolarized by approximately 20 mV via depolarizing current injection (to mimic the NMDA-induced depolarization), 5HT increased rather than decreased membrane conductance. Furthermore, 5HT did not affect the increase in membrane conductance following NMDA applications in zero Mg2+ saline. The results suggest that intrinsic, NMDA receptor-mediated voltage oscillations develop in a brief period after hatching, and that they depend upon the co-activation of 5HT and NMDA receptors. The enabling function of 5HT may involve the facilitation of the voltage-dependent block of the NMDA ionophore by Mg2+ through activation of receptors with 5HT1alpha-like pharmacology.
在晚期胚胎和早期幼体非洲爪蟾的运动神经元中，研究了内在的N - 甲基 - D - 天冬氨酸（NMDA）受体介导的电压振荡的发展及其对5 - 羟色胺（5HT）受体共激活的依赖性。在河豚毒素存在的情况下，100微摩尔的NMDA引起约20毫伏的膜去极化，但未导致电压振荡。然而，加入2 - 5微摩尔的5HT后，在12%的胚胎运动神经元和70%的幼体运动神经元中观察到了振荡。电压振荡依赖于NMDA和5HT受体的共激活，因为用D - 2 - 氨基 - 5 - 膦酰基戊酸（APV）选择性阻断NMDA受体或从实验盐溶液中排除Mg2+可使其减弱。在没有NMDA的情况下应用5HT也未能引发振荡。非选择性的5HT1α受体激动剂5 - 羧酰胺色胺（5CT）可诱导振荡，并且5HT和5CT诱导的振荡都被选择性的5HT1α受体拮抗剂pindobind - 5HT1消除。为了测试5HT是否通过调节Mg2+对NMDA通道的电压依赖性阻断来实现电压振荡，在河豚毒素存在的情况下监测膜电导。尽管5HT引起4 - 8毫伏的膜超极化，但电导几乎没有可检测到的变化。应用NMDA引起约20毫伏的去极化和电导的“明显”降低，推测是由于电导脉冲使膜进入Mg2+阻断NMDA离子通道的电压区域。5HT进一步降低了电导，我们认为这是由于其增强了电压依赖性Mg2+阻断。当通过注入去极化电流使膜电位去极化约20毫伏（以模拟NMDA诱导的去极化）时，5HT增加而不是降低膜电导。此外，5HT不影响在零Mg2+盐溶液中应用NMDA后膜电导的增加。结果表明，内在的、NMDA受体介导的电压振荡在孵化后的短时间内发展，并且它们依赖于5HT和NMDA受体的共激活。5HT的促成功能可能涉及通过激活具有5HT1α样药理学特性的受体来促进Mg2+对NMDA离子通道的电压依赖性阻断。
